1. Field of the Invention
The present invention relates to a method and an apparatus for limiting the capacity of at least a hydrostatic machine.
2. Description of the Prior Art
A secondary control system basically comprises a primary unit, i.e. a hydrostatic machine of an adjustable delivery volume which is operative as a pump, which machine supplies pressurized fluid to a pressure line in which an impressed pressure is maintained and comprises further a secondary unit, i.e. a hydrostatic machine of an adjustable displacement volume which is connected to the pressure line to either drive a load in being operative as a motor, or being driven by a load to be operative as a pump returning pressurized fluid into the pressure line.
The principle of the secondary control is generally based on the assumption that the pressure line has an unlimited capacity, i.e. the delivery volume of the pump or pumps supplying fluid to the pressure line is larger or at least equal to the maximum suction volume of the secondary unit or, respectively, a number of secondary units, wherein the energy for the hydraulic adjustment of the machines as well as the system efficiency is incorporated in the calculation. Pressure deviations in a predetermined range are considered to be permissible so that a so-called impressed pressure which must not necessarily always be constant is maintained in the pressure line.
However, when the maximum pump delivery capacity defining the primary capacitor is lower than the maximum volume comsumption of the secondary units, these units may suck the pressure line empty, the pressure breaks down and the speed of the secondary unit as well as the volume capacity thereof must be reduced until the primary unit can re-establish the impressed pressure.
Systems are known which are disclosed in German patent applications 34 41 185, 35 08 339 and 37 13 799 for example, in which the pressure in the pressure line is measured and the desired value for the speed adjustment is reduced as soon as the pressure starts to decrease.
In the operation of control systems of this type the volume capacity of the secondary unit resulting does not directly depend on the desired speed value, but is adjusted depending on external conditions so that the desired speed value must be continuously re-adjusted. Methods of this type which indirectly adjust the volume capacity of the secondary unit are expensive and tend to instabilities.